1. Field of the Invention
The present invention relates to aqueous polyurea dispersions which have improved hardness and solvent resistance due to the incorporation of aldimines into the polymer backbone, to a process for their preparation and to their use for the production of coating compositions.
2. Description of the Prior Art
The production of linear or cross-linked aqueous polyurethane-urea dispersions is known as shown by U.S. Pat. Nos. 3,479,310; 4,066,591; 4,092,286; 4,108,814; 4,237,264; and 4,238,378 which disclose linear polyurethane-ureas and U.S. Pat. Nos. 3,870,684 4,203,883 and 4,408,008, which disclose cross-linked polyurethane-ureas. The aqueous polyurethane-urea dispersions may be used for a wide range of commercial applications such as adhesives or coatings for various substrates including textile fabrics, plastic, wood, glass fibers and metals. Chemical resistance, abrasion resistance, toughness, tensile strength, elasticity and durability are among the many desirable properties of these coatings. In some cases these properties of coatings prepared from aqueous polyurethane-urea dispersions have equaled or even surpassed the performance levels of coatings obtained from solvent-based polyurethane lacquers.
Regardless of the property level which may be obtained for coatings prepared from commercial polyurethane-urea dispersions, there is always a further need to increase these properties, in particular hardness and solvent resistance.
It is an object of the present invention to provide aqueous polyurea dispersions which can be used to prepare coatings which possess improved properties, especially hardness and solvent resistance.
This object may be achieved in accordance with the present invention by preparing the aqueous polyurea dispersions by incorporating aldimines into the polymer backbone, preferably during the preparation of an NCO prepolymer.
The use of ketimines and aldimines for preparing aqueous polyurethane-urea dispersions has been disclosed in U.S. Pat. No. 4,829,122. In accordance with this patent NCO prepolymers and either aldimines or ketimines are mixed and dispersed in water. Upon contact with water the aldimines and ketimines are hydrolyzed to the corresponding amine, which then chain extends the NCO prepolymer to form the dispersed polyurethane-urea. This reference does not disclose any direct reaction between an aldimine and an NCO prepolymer and clearly does not disclose chemically incorporating an aldimine into the NCO prepolymer without hydrolysis of the aldimine.